Wangari Maathai speaks during an interview held in Dar es Salaam, Tanzania, in East Africa. Ms. Maathai began her Green Belt Movement in the 1970s to encourage African villagers to plant trees. Today, her Billion Tree Campaign has resulted in the planting of 11 billion trees worldwide.

Radu Sigheti/Reuters

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New York

On a visit to Japan, Wangari Maathai learned the story of the hummingbird and the forest fire. While the other animals run in fear or hang their heads in despair, the hummingbird flies above the fire time and again, releasing a few drops of water from its tiny beak.

"Why do you bother?" the other animals shout at the hummingbird. "I'm doing the best that I can," the hummingbird replies.

"It's such a beautiful story," Ms. Maathai says, thinking of the immensity of the world's environmental problems. "There is always something we can do with our little beak like the little hummingbird."

In 2004 Maathai was honored with a Nobel Peace Prize for her work founding the Green Belt Movement, which enlists villagers, and especially women, to improve their local environment.

Since then, she's concluded that people's values are what motivate them. If the values are good ones, good actions will follow. Hence it's importance for people to tap their spiritual traditions for guidance in caring for the environment, she says.

"I saw that if people have [good] values, they can sustain what they are doing," says Maathai in a recent interview at a New York City hotel not far from the United Nations, where she's addressed the General Assembly in the past.

"If you don't have good values, you'll embrace vices," she says. And if we give in to the vices, "We destroy ourselves. We destroy the environment. If we can embrace [good] values, we also heal ourselves. And in the process we heal the environment."

That's the message of her new book, "Replenishing the Earth: Spiritual Values for Healing Ourselves and the World."

People learn these values from their parents, their teachers, and their religious leaders, says Maathai, whose own background is Christian.

In the 1970s, her call for African villagers to plant trees may have seemed like a simplistic response to complex environmental problems. But she realized trees hold both symbolic and practical value.

"People use trees for so many things. It was easy to get into a community to talk about trees," Maathai recalls. They provide immediate practical resources: food, firewood, and shelter. They fight soil erosion.

Today, her Billion Tree Campaign has resulted in more than 11 billion trees being planted worldwide.

Her Green Belt Movement embraces four key values: love for the environment, gratitude and respect for Earth's resources, self-empowerment and self-betterment, and the spirit of service and volunteerism.

The Green Belt followers, many of them women, take simple actions in their communities, such as tending tree nurseries, terracing their fields to curb erosion, collecting rainwater, planting home gardens, and building low-tech sand dams.

Maathai attributes her ideas as having come from "the Source" or, in a Christian context, God. But listening for ideas must also be accompanied by "an attitude that allows you to take advantage of that awakening," she writes in "Replenishing the Earth." "This entails keeping your mind, eyes, and ears open, so that when an idea arrives you'll be ready for it."

Maathai is critical of some aspects of Christianity's influence on Africa – especially theologies that suggest suffering is inevitable and that relief will come only in the next world – a view that can lead to resignation and defeatism. "I don't think God puts us here on this planet to suffer or to do nothing so that we suffer," she says.

But she's heartened that many religious traditions, including Christianity, teach that humans should be good stewards of the environment.

"People of faith ought to be in the forefront protecting this creation," says Maathai, who has also written a book on her home continent ("The Challenge for Africa") and a memoir ("Unbowed"). "I'm glad to see that even the pope has come up front to make statements in favor of the environment," she says.

She's now back in her homeland of Kenya beginning work on the Wan­gari Maathai Institute for Peace and Environ­mental Studies at the University of Nairobi. "They want to call it after me, and I'm very flattered," she says. "They have given me 50 acres of land" on which to build.

Her institute will emphasize "learning by doing," she says, applying knowledge to real problems. "What Africa needs is people who are willing to get their fingers dirty and work with the people."

Planting trees is one way people connect with the natural world, she says. "For unless we see [nature], smell it, or touch it, we tend to forget it, and our souls wither," she writes in "Replenishing the Earth."

She continues to inspire others.

"Wangari Maathai is a unique presence on our planet," says Mary Evelyn Tucker, cofounder of the Forum on Religion and Ecology at Yale University, in an e-mail. "She exudes love and joy in all she does. Her words and deeds manifest to us that Earth is a sacred community including humans, ecosystems, and all species....

"She reminds us that there is no lasting peace until we have peace with the Earth itself."